The present invention relates to an anode active material comprising, for example, an alloy material (including an intermetallic compound) capable of electrochemically reacting with lithium (Li), and more specifically relates to an improvement in cycle characteristics of the anode active material. Moreover, the present invention relates to a method of manufacturing the anode active material and a nonaqueous electrolyte secondary battery using the anode active material.
In recent years, a large number of portable electronic devices such as camcorders, cellular phones and laptop computers have been emerged, and the size and the weight of them have been reduced. Research and development aimed at improving the energy densities of batteries used as power sources of the electronic devices, specifically secondary batteries as a key device have been actively promoted. Among the batteries, a nonaqueous electrolyte secondary battery (for example, a lithium-ion secondary battery) can obtain a high energy density, compared to a conventional aqueous electrolyte secondary battery such as a lead-acid battery and a nickel cadmium battery, so the improvement of the battery has been studied in all quarters.
As an anode material used in the lithium-ion secondary battery, a carbon material having a relatively high capacity and superior cycle characteristics such as non-graphitizable carbon or graphite is broadly used. However, in consideration of a recent demand for a higher capacity, a further increase in the capacity of the carbon material presents a challenge.
In such a background, a technique of achieving a carbon material with a high capacity through selecting a material to be carbonized and forming conditions has been developed (for example, refer to Japanese Unexamined Patent Application Publication No. Hei 8-315825). However, when such a carbon material is used as an anode material, an anode has a discharge potential vs. lithium of 0.8 V to 1.0 V, and when a battery includes the carbon material, the discharge voltage of the battery is reduced, so a significant improvement in the energy density of the battery cannot be expected. Moreover, there is a disadvantage that the hysteresis in the shape of a charge-discharge curve is large, thereby energy efficiency in each charge-discharge cycle is low.
On the other hand, as an anode with a higher capacity than the carbon material, an alloy material which is formed through electrochemically alloying some kind of metal with lithium and is reversibly produced and decomposed has been researched. For example, an anode with a high capacity using a Li—Al alloy has been developed, and an anode with a high capacity including a Si alloy has been developed (for example, refer to U.S. Pat. No. 4,950,566).
However, the Li—Al alloy or the Si alloy has a big problem that the cycle characteristics are extremely poor, because the alloy expands or shrinks according to charge and discharge, so every time a charge-discharge cycle is repeated, the anode is pulverized.
Therefore, in order to improve the cycle characteristics, a technique of coating the surface of an alloy material with a material with high conductivity has been considered (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2000-173669, 2000-173670 and 2001-68096). In techniques described in the above patent literatures, the surface of the alloy is coated with a conductive material through a technique of immersing the alloy material in an organic solvent in which a conductive material is dissolved, or a technique of using a mechanochemical reaction such as hybridization, thereby the cycle characteristics are improved.
However, even in the case where these techniques are used, an effect of improving the cycle characteristics are not sufficient, so the fact is that advantages of the anode with a high capacity including the alloy material cannot be used sufficiently.